Device for forming a compound oxide superconductor thin film

ABSTRACT

There is disclosed a device for forming a thin film on a substrate by irradiating a target of a compound oxide superconducting material with a laser beam and evaporating on the substrate a thin film corresponding to a composition of the target in an oxygen ambient atmosphere by laser evaporation, a scanning optical system for causing the laser beam to scan being disposed in an optical path of the laser beam.

This application is a continuation of application Ser. No. 07/738,036,filed Jul. 31, 1991, now abandoned, which application is entirelyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a device for forming a compound oxidesuperconducting thin film. Specifically, the invention relates to adevice for forming a compound oxide superconducting thin film by laserevaporation.

2. Prior Art

Superconductivity phenomenon, which is said to be related to a phasetransition of electrons, has been recognized as a phenomenon observedonly at very low temperatures whose realization requires liquid helium.But in 1986 Bednorz, Muller et al. discovered (La,Ba)₂ CuO₄, whichexhibits superconductivity at 30K. In 1987 Chu et al. discovered thatYBa₂ Cu₃ O_(y) has a superconductivity critical temperature Tc in theorder of 90K. In 1988 Maeda et al. discovered the so-called Bi-systemcompound oxide superconducting material having a critical temperaturehigher than 100K. These compound oxide superconducting materials canexhibit superconductivity phenomenon even with cooling by inexpensiveliquid nitrogen. The potential of the practical applicability ofsuperconduction technology has been noted.

Compound oxide superconducting materials having the above-described highcritical temperatures were originally prepared in sintered bodies bypowder metallurgy. But the sintered bodies could not exhibit preferablecharacteristics, especially critical current density, etc. Recentlyprocesses for forming superconducting materials in thin films have beenstudied. Usually compound oxide superconducting thin films are formed onSrTiO₃ mono crystal substrates, MgO mono crystal substrates, etc., byvarious kinds of evaporation, such as vacuum evaporation, spattering,etc.

The compound oxide thin film formed by various kinds of conventionallyknown evaporation techniques do not generally exhibit sufficientsuperconductivity as formed. To make them useful superconducting thinfilms, it is necessary to post-anneal the thus-formed compound oxidethin films. That is, it is generally known that compound oxidesuperconducting materials exhibit high oxygen non-stoichiometry, andalso that the lower non-stoichiometry they have, the higher thesuperconductivity they exhibit. In conventional superconducting thinfilms, the post-annealing is performed for the purpose of supplementingoxygen due to the oxygen non-stoichiometry.

But it is known that when a thin film formed on a substrate is annealed,the substrate material is diffused into the thin film, and in the regionnear the substrate, the quality of the superconducting thin film is muchdegraded. The thus-treated thin film is usable only at a portion nearthe surface for experimental purposes, but it is not usable forpractical purposes, such as production of various devices, etc. Underthese circumstances, laser evaporation, which has not been so far noted,is suddenly noted.

Laser evaporation comprises irradiating a target with a high power laserbeam, evaporating material from the target, and depositing the same on asubstrate. The heating of the target and the substrate can be controlledindependently of each other. It is not always necessary to make theambient atmosphere highly vacuum. It is possible to increase the filmforming speed by selecting suitable conditions. In addition, when acompound target is used, little composition change is observed betweenthe composition of the target and that of the thin film which is formed.Thus, laser evaporation is considered suitable to form compound oxidesuperconducting materials in thin films. It has been confirmed that thecompound oxide superconducting films formed by laser evaporation withoutthe above-described post-annealing step exhibit useful superconductivecharacteristics. Laser evaporation is expected to be dominant inprocesses for forming oxide superconducting films. In theabove-described laser evaporation process, when a laser beam irradiatesa target, a blaze-like thing is generated. This blaze-like thing iscalled a plume. This is a collection of active materials generated fromthe surface of the target by the irradiation of the laser beam, and itis deposited on a substrate, so that a thin film is formed.

But a sectional area to be contributed to the film formation by theplume is generally smaller than the area of the substrate. A thin filmis formed by laser evaporation only on a part of the film formingsurface of the substrate. On a typically substrate having a size ofabout 20 mm×20 mm, a thin film which can exhibit effectivesuperconductive characteristics is formed in about a 10 mm-diameter atmost. Such small thin films are not practical.

SUMMARY OF THE INVENTION

This invention relates to a device for forming a thin film by laserevaporation. An object of this invention is to provide a device forforming a thin film, especially a thin film of a compound oxidesuperconducting material, on a substrate having a larger film formingsurface.

It is an object of the present invention to provide a device for forminga thin film on a substrate by irradiating a target of a compound oxidesuperconducting material with a laser beam and evaporating a part of thetarget, and forming a thin film corresponding to the composition of thetarget on the substrate in an oxygen ambient atmosphere by laserevaporation. A scanning optical system for causing the laser beam toscan is disposed in an optical path of the laser beam.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of the applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of an arrangement of the device for forming asuperconducting thin film according to a first embodiment of thisinvention;

FIG. 2 is a view of an arrangement of a substrate transfer device usedin the device of FIG. 1;

FIG. 3 is a plan view of a test-formed large area-thin film; and

FIG. 4 is a pictorial view of an arrangement of the device for forming asuperconducting thin film according to a second embodiment of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The device according to this invention is a device for forming acompound oxide superconducting thin film by laser evaporation. Theinvention is characterized by a scanning optical system such as a rotarypolygon mirror and a typical unit used in a film forming device.

That is, the device according to this invention has realized that if alaser beam is caused to scan the target by, e.g., a rotary polygonmirror, a larger area superconducting thin film is formed than the thinfilm layer area formed by a fixed laser beam. But, in the case where thescanning direction of a laser beam by the scanning optical system is onedimensional, it is preferable to transfer the substrate in a directionperpendicularly to the direction of scanning of the laser beam, tothereby form the film on the substrate. Thus a superconducting thin filmcan be formed over a required area. The scanning optical system may beso arranged that a laser beam can be caused to scan two-dimensionally.This embodiment is shown in FIG. 4. In this embodiment, a firstgalvanometer mirror deflects a laser beam in one direction, and a secondgalvanometer mirror causes the laser beam to scan in a directionperpendicular thereto, whereby two-dimensional scanning can beperformed. This arrangement according to the invention does not requiretransfer of the substrate. This arrangement enables a laser beam spot ona target to scan over the target, and as the result, local consumptionof the target can be prevented. In other words, one target can lastlonger, and substantial consumption of a target can be decreased.

This invention is applicable to all thin film forming techniques usinglaser evaporation, and the superconducting thin films to which thisinvention may be especially effectively applied are oxidesuperconducting materials including La--Ba--Cu, Y--Ba--Cu, Bi-- andTl-system oxides, which are formed on SrTiO₃ or MgO single-crystalsubstrate, etc.

FIG. 1 is a pictorial view of an arrangement of the device for forming athin film according to a first embodiment of this invention. Theinterior of the film forming chamber is shown in a sectional view inFIG. 1.

As shown in FIG. 1, this device mainly comprises a film forming chamber8 which houses a substrate holder 2 for holding a substrate 1 and atarget holder 4 for holding a target 3. The device further and has anexhaust port 5, an oxygen supply port 6 and a laser beam entrance window7. A laser irradiating means means is included with the devices. Thelaser irradiating means includes a laser 11 which has an optical system10 and generates a laser beam, and a rotary polygon mirror 12.

The substrate holder 2 is set on a translating device 20 for thesubstrate holder 2, which will be explained later, and the substrateholder 2 is movable back and forth with respect to the sheet of FIG. 1.The oxygen supply port 6 is connected to a nozzle 6a so that oxygen gasis blown near a film forming surface of the substrate 1.

As shown in FIG. 1, the oxygen supply port 6 and nozzle 6a are arrangedso as to blow oxygen gas (O₂) onto and along the surface of thesubstrate. The optical system 10 converges a laser beam emitted from thelaser 11 or expands the beam radius of the laser beam to supplement itsoutput power. The optical system 10 is designed suitably to meetnecessary film forming requirements.

FIG. 2 shows a structural example of the transfer device 20 for thesubstrate holder 2 shown in FIG. 1. In FIG. 2, the transfer device 20 islaid on a side with a film forming surface of the substrate facingoutward.

As shown in FIG. 2, the transfer device 20 mainly comprises a frame 21having a predetermined sectional shape, and a plurality of pairs ofguide rollers 22 with the rollers of each pair opposed to each other.The guide rollers 22 are arranged so as to engage in a pair of guidegrooves 2a cut in both side walls of the plate-shaped substrate holder2. Some of the guide rollers 22 are forcedly driven by driving means 24.The driving means 24 may be reversible to thereby reverse a transferdirection.

In the frame 21, there is provided a heater 23. The heater 23 comprisesa plurality of rod-shaped heating bodies 23a arranged perpendicular to atransfer direction. Each of the rod-shaped heating bodies 23a can beindividually controlled, so that temperature distributions can be formedin a transfer direction of the substrate holder 2.

The transfer device 20 of the above-described structure is used asfollows. The substrate holder 2 including a substrate 1 set on isinserted into the frame 21 so that the guide rollers 22 are engaged intoa pair of guide grooves 2a. The frame 21 is placed at a set position inthe film forming chamber of FIG. 1. The driving means 24 is actuated totransfer the substrate 2 linearly. At this time, the heater 23 is turnedon to heat the substrate 1 up to a required temperature. For accuratetemperature control, it is preferable to make the substrate holder 2 ofa metal or ceramic having good heat conductivity.

In the device for forming a thin film having the above-describedstructure, the film forming operation is explained as follows.

A substrate 1 and a target 3 are set on the substrate holder 2 and atarget holder 4, respectively, and then the film forming chamber 8 isdegassed. Subsequently, with the rotation the rotary polygon mirror 12,the laser 11 is actuated to irradiate a laser beam onto the target 3.The transfer device 20 is moved during the irradiation of the laserbeam, so that plume 9 generated from the target 3 radiates all over thesubstrate 1. During the film forming, oxygen gas is blown from thenozzle 6a onto a film forming surface of the substrate 1. By thisoperation, a uniform thin film can be formed on the entire surface ofthe substrate 1.

The inventors actually formed Y--B--C--O system thin films using thesame device as shown in FIGS. 1 and 2. An Ar--F pulse laser oscillatorhaving an output energy density of 2 J/cm² on the surface of the targetand 10 Hz frequency was used as the laser beam source. As the target, asintered body of YBa₂ Cu₃ O_(x) having a size of 15 cm×3 cm×1 cm wasused. As the substrate, 2×3 array sheets of MgO(100) having a size of 5cm×5 cm substrates were located as shown in FIG. 3. The distance betweenthe substrate and the target was set to be 5 cm. The interior of thefilm forming chamber was kept at a vacuum of 200 mTorr. Oxygen was blownonto the substrate from the nozzle 6a at the rate of 30 cc/min. The filmforming temperature was 700° C. The polygon mirror had 12 planes. Therotation speed was 5 rpm. The thin film forming was conducted for 50minutes under these conditions. On Positions (a) to (f) in FIG. 3, thethickness of the film (Angstrom: Å), the critical temperature Tc (K),and the critical current density Jc (A/cm²) were measured. The followingis the resultant data.

    ______________________________________                                        Positions                                                                              Thickness (Å)                                                                           Tc(K)   Jc(A/cm.sup.2)                                 ______________________________________                                        a        800           89      5.0 × 10.sup.6                           b        780           88      4.0 × 10.sup.6                           c        820           89      5.5 × 10.sup.6                           d        780           88      4.8 × 10.sup.6                           e        790           87      4.2 × 10.sup.6                           f        820           89      5.2 × 10.sup.6                           ______________________________________                                    

It was found that by using this embodiment, a homogeneous, good compoundoxide superconducting thin film can be formed over even an area as largeas about 15 cm×10 cm.

The device for forming a thin film according to a second embodiment ofthis invention will be explained below with reference to FIG. 4.

The arrangement of FIG. 4 is the same as that of the first embodiment ofFIGS. 1 and 2 in that a substrate 1 is held by a substrate holder 2, atarget 3 is held by a target holder (not shown), and these holders arehoused in a film forming chamber 8 with a laser beam entrance window 7.It is also the same as in the first embodiment that an oxygen supplynozzle (not shown) is disposed near the substrate 1, and a laser beamfrom a laser beam source 11 is condensed by a lens 10. The followingthree points are differences of the second embodiment from the firstone.

Firstly, in place of the rotary polygon mirror, a galvanometer mirror 40is used as an optical system for the scanning of a laser beam (scanningin the Y direction on the target). The galvanometer mirror 40 comprisesa plane mirror 41 inserted in an optical path of the optical system, anda shaft 42 fixed to the center of the plane mirror 41. Anelectromagnetic reversible driving means 43 is connected to the shaft42. The galvanometer 40 is driven reversibly within a set range in thedirections indicated by the arrows in FIG. 4, so that the incident angleof a laser beam with respect to the plane mirror 41 is varied, and thespot of the laser beam is caused to scan in the direction Y on thetarget 3.

Secondly, in addition to the galvanometer mirror 40 for the scanning inthe direction Y, there is provided a galvanometer mirror 50 fordeflecting a laser beam in the direction X. The galvanometer 50comprises a plane mirror 51, a shaft 52, and a reversible driving means5S. Thus, it is possible that the spot of a laser beam on the target 3is caused to scan in the direction X by the galvanometer 50 while beingcaused to scan by the galvanometer 40 on the target 3 in the directionY. Accordingly in the second embodiment a mechanism for transfer thesubstrate holder 2, e.g., the transferring means of FIG. 2, is notnecessary. This is a third difference of the second embodiment from thefirst one.

In forming a thin film using the device according to the secondembodiment, while the mirror 40 for the scanning in the direction X isbeing driven, the mirror 50 for the scanning in the direction Y isdriven at a relatively low speed. At this time, the target 3 does nothave to be moved. The target 3 is irradiated with a laser beam over alarge area, and accordingly the consumption of the target can beminimized. Furthermore, a homogeneous, high-quality superconducting thinfilm can be formed over a large area of a stationary substrate 1.

This invention is not limited to the above-described embodiments, butrather, it covers various modifications. To give an example, the opticalsystem for causing a laser beam to scan is not limited to the opticalsystem of the type in which a reflecting mirror is movable, but instead,an optical system of the type in which the optical refractive index isvariable may be used. For example, electrooptic crystals, such asLiNbO₃, can vary the refractive index by an electric field. It ispossible to cause a laser beam to transmit through these crystal whichhave electrodes so that the laser beam is caused to scan in theabove-mentioned directions X and Y by applying a voltage to theelectrodes. The deflection of a laser beam in the direction X can beperformed by moving the laser beam source 11, because the scanning inthe direction X is slower than in the direction Y. In short, the deviceaccording to this invention is characterized in that a substrate for athin film to be formed on is moved at a low speed in the direction Xwhile causing the laser beam to scan in the direction Y by an opticalsystem at a relatively high speed, or the optical axis of a laser beamis moved (or deflected) in the direction X at a low speed. Thesecharacteristics may be practiced by any means.

The superconducting thin film formed by laser evaporation has littlediffusion of a substrate material into a superconducting thin film.Therefore, the superconducting film is of high quality, because laserevaporation can omit post-annealing from the thin film forming process.The oxide superconducting thin film formed by the device according tothis invention is suitably usable in various devices, such as a stringof thin film devices, superconducting quantum interference devices(SQUIDs), etc.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

We claim:
 1. A device for forming a compound oxide superconducting filmon a substrate, comprising:a holding means for holding a target made ofa compound oxide superconducting material; a laser beam irradiatingmeans for irradiating the target by a laser beam to evaporate a thinfilm corresponding to the oxide superconducting material of the targetonto the substrate; means for scanning the laser over the target in aone dimensional scanning direction; transfer means for moving thesubstrate in a transfer direction, wherein the transfer direction isperpendicular to the one-dimensional scanning direction, wherein saidtransfer means includes (a) a frame having plural pairs of guide rollersarranged such that the guide rollers of each pair are arranged oppositeto one another, (b) a substrate holder, wherein the frame and substrateholder are movable with respect to one another in said transferdirection, and (c) a heater for heating the substrate to be heldthereon; and an oxygen supply means disposed proximate the substrate forsupplying O₂ gas on a surface of the substrate on which the oxidesuperconducting film is to be formed; whereby laser beam evaporation isperformed in an oxygen ambient atmosphere.
 2. A device according toclaim 1, wherein said pairs of guide rollers engage in guide groovesdefined in opposed sides of the substrate holder.
 3. A device accordingto claim 2, wherein the heater includes a rod shaped heating bodyarranged perpendicular to said transfer direction.
 4. A device accordingto claim 2, wherein the heater includes a plurality of rod shapedheating bodies arranged perpendicular to said transfer direction.